1. Field of the Invention
The present invention relates to a process for producing a photovoltaic element like solar cells and optical sensors, particularly to a process for producing a photovoltaic cell where a buffering semiconductor layer is formed at the interface in the semiconductor junction.
2. Related Background Art
Solar cells utilizing the photovoltaic effect of a semiconductor as a photovoltaic element are being actively studied and developed for commercialization, since the solar cells utilize natural energy of sunlight. There are many types of solar cells in classification by the semiconductor material. For example, a solar cell employing a non-monocrystalline semiconductor film is produced generally by a plasma CVD (chemical vapor deposition) process.
The semiconductor layers of the solar cell have a semiconductor junction such as a pn junction, and a pin junction. In the case where thin films of a-Si or the like is employed as the semiconductor layer, a desired semiconductor junction can readily be formed by laminating, on a substrate successively, semiconductor films produced by mixing a source gas such as silane (SiH.sub.4), and a dopant element such as phosphine (PH.sub.3) and diborane (B.sub.2 H.sub.6), and decomposing the mixture by plasma activation to form a semiconductor film of a desired conductivity type. In producing a solar cell of a non-monocrystalline type, the respective semiconductor layers are formed in separate film formation chambers.
U.S. Pat. No. 4,400,409, for example, discloses a continuous plasma CVD apparatus employing a roll-to-roll system. This apparatus has plural glow discharge region, namely chambers for formation of semiconductor layers, in series, and a flexible very long substrate sheet having a prescribed breadth is allowed to pass through the chambers successively. With the passage of the substrate, a prescribed conduction type of semiconductor layer is deposited in the glow discharge region of the respective chambers onto the substrate. By continuous delivery of the substrate in the length direction, elements having semiconductor junctions are produced continuously. In this apparatus, gas gates are provided to prevent the diffusion of the dopant gas for the film formation reaction into the adjacent glow discharge region. Specifically, the respective glow discharge regions are separated by a narrow slit-shaped opening as a separation path, and the separation path is swept by a gas such as Ar, and H.sub.2.
However, such conventional processes for producing photovoltaic elements (solar cells) have disadvantages below.
(1) A first disadvantage is the low initial photoelectric conversion efficiency. For example, during or after formation of an i-type layer on an n-type layer, phosphorus (P), an n-type layer dopant, diffuses thermally into the i-type layer to weaken the ni junction to drop the open-circuit voltage or fill factor, although the contamination of the gaseous dopant is prevented by separation of the i-type layer formation space from the p- or n-type layer formation space by the gas gate.
(2) Another disadvantage is the low reliability owing to deterioration of the solar cells caused by gradual thermal diffusion of a dopant for a p-type layer or an n-type layer into the i-type layer during practical use under a varieties of conditions of weather and installation.
The solar cell is required to have sufficiently high photoelectric conversion efficiency and stable properties, and to be producible readily industrially. Therefore, solar cells are desirably produced industrially with reproducibility at a high film formation rate with a large cell area with improvement of electrical, optical, photoconductive, and mechanical properties, fatigue strength, and environmental resistance.
Power generation systems employing solar cells are usually provided by connecting standardized unit modules of a solar cell element in series or in parallel into a unit to obtain a desired voltage and electric current.
In the unit construction, the overall properties of the unit are depend on the unit module of the lowest current-voltage properties. Therefore, it is important not only to improve properties of the respective unit modules but also to reduce variation of the properties among the unit modules. Moreover, not to lower the production yield, wire disconnection and short-circuit should be prevented. For this purpose, the semiconductor layers, which are the main portion of the solar cell, should be made uniform in properties in production of the unit modules to decrease defects of the semiconductor layers. Therefore, in consideration of facility of the module design and simplification of the module assembling process, it is necessary to provide a semiconductor deposition film having uniform properties over a large area in order to raise the productivity and reduce remarkably the production cost of the solar cells.